You can find revamped version of this mod for 1.18+ here

Inspired by various Herobrine projects, the mod adds a powerful new enemy into the game. This version of Herobrine is perfect for those who prefer a more vanilla-like experience without the need to bother with tons of new items and summoning mechanics.

Main features:

Adds a new boss mob called Pursuer who restlessly intrudes into personal space of a random player in the world roughly once per day. This mysterious figure is completely neutral initially, but can be engaged in combat. If you dare.

Only one Pursuer can exist in each world and will not come back once defeated without the use of commands. However, you should prepare for a really bad time as he will not go down in a fight that easily and is capable of escaping even Death herself. As if that wasn't enough, the Pursuer grows angrier and more impatient every time he sustains a wound (beaten in combat)

Several new advancements are added to track your progress against the boss. Defeating the Pursuer for good rewards the victor with a new consumable item which grants massive boost to health and damage as long as the user remains alive.

Detailed review of the boss (contains spoilers!):

The Pursuer is a fast and strong boss capable of teleportation who is difficult to defeat alone without high-tier gear and consumables. His behavior can be divided into two stages: Observation and Combat. Said stages become more challenging every time you win a battle against the boss. In addition, the Pursuer changes his weapon and receives boosts to various stats.

During Observation stage the Pursuer teleports to a random position somewhere around chosen player and warps away if either the players are too close or too far or enough time passes. After the boss sustains a minor wound, getting too close or waiting for too long is no longer an option and the only way to escape is to run away as fast as you can. But even running away becomes unsafe once a major wound is inflicted. However, there are still ways to avoid fighting. For example, the Pursuer really dislikes water and will leave upon staying in it for too long.

Combat stage begins once the boss is damaged or provoked as described above. The Pursuer is initially equipped with an Iron Sword and will not hesitate to use it in close quarters. He has lower health than vanilla bosses, but is capable of dodging incoming attacks through teleportation. Inflicting wounds adds new moves to the list such as summoning reinforcements, placing TNT (can be disabled through 'mobGriefing' game rule) and more.

Game rules and commands:

Game rule 'pursuerChanceMultiplier' controls the chance of the Pursuer to appear near a random player. Default value is 5. Setting it to 0 completely prevents the boss from appearing while very high values such as 5000 cause him to appear almost instantly (the latter should mainly be used for practice and showcase as the game can get laggy). It should be noted that the Pursuer ignores players in Creative and Spectator game modes.

Game rule 'pursuerConditionalSpawning' prevents the wounded (stronger) versions of the Pursuer from appearing until at least one of the players progresses far enough into the game. Default value is OFF, it allows all versions of the boss to spawn without their respective restrictions as long as the previous version is defeated. One of the players needs to get the following advancements to progress the fight if the value is set to ON:

1) 'Minecraft';

2) 'Nether' or 'The End?';

3) 'Eye Spy' or 'Free the End' or 'The Next Generation' or 'Bring Home the Beacon'.

Game rule 'pursuerRestTime' sets the minimum number of ticks that needs to pass for the Pursuer to spawn again after being defeated in combat (but not killed). Default value is 36000 (at 20 ticks per second = 30 minutes).

Game rule 'pursuerIsAngry' makes the Pursuer instantly attack players upon spawning if set to ON. Default value is OFF.

Cheat command 'pursuerstatus' shows the current level of the boss (increases with each wound inflicted, 0 means he is defeated for good) and whether or not he stalks or fights someone in the world at the moment.

Cheat command 'pursuerincrementlevel' increases Pursuer's level and resets back to 0 if maximum level is already reached (can be used to revive the boss).

Cheat command 'pursuercanspawnforplayer' shows whether the current player fulfills their conditions for the Pursuer to spawn nearby. This command does not check the world's difficulty, but does check the player's game mode and conditions from the 'pursuerConditionalSpawning' game rule.

Cheat command 'pursuerresetspawnchecks' resets all checks from the 'pursuerConditionalSpawning' game rule. This command should be used after removing advancements from all players for the changes to take effect.

The mod is created with the help of MCreator. Credit for some of the textures goes to anonymous users from Novaskin.

The Pursuer (Herobrine-themed boss mod)